Application of quaternary onium salts
Quaternary onium salts are useful substances as phase transfer catalysts and extracting agents for various anions in an aqueous phase using their ion exchange-ability.
When the quaternary onium salts are tried to be used in the fluorine chemistry, the quaternary onium cation of .sup.+ AR.sup.1 R.sup.2 R.sup.3 R.sup.4 (hereinafter referred to as "Q.sup.+ ") in formula [I] and the polyfluorocarboxylate anion of RfCO.sub.2.sup.- (hereinafter simply referred to "RfCO.sub.2.sup.- ") form a very stable ion pair in a hydrophobic organic solvent and it was found to be difficult to ion exchange this RfCO.sub.2.sup.- with another anion. Thus, it is necessary to exchange the RfCO.sub.2.sup.- with an easily ion exchangeable anion by some method since in the extraction or the phase transfer catalytic reaction RfCO.sub.2.sup.- Q.sup.+ as such cannot repeatedly be used. For example, in the fields as will be described below, the development of techniques for converting the quaternary onium fluorine-containing carboxylate of formula [I] to an easily ion exchangeable quaternary onium salt is demanded.
(1) Recovery of polyfluorocarboxylic acids
Polyfluorocarboxylic acids are useful compounds as fluorine-containing surfactants and solvents and are very expensive, and an effective method of recovering them is desired but at present no excellent method is found. As described above, since the polyfluorocarboxylate anion is confirmed to form a very stable ion pair with Q.sup.+ in an organic phase, it is possible to simply extract the polyfluorocarboxylate anion as the counter ion to Q.sup.+ in the organic phase by contacting an aqueous solution or an emulsion containing a polyfluorocarboxylate salt with an organic phase containing Q.sup.+.
However, an effective method for removing the polyfluorocarboxylate anion from a stable ion pair of Q.sup.+ and a polyfluorocarboxylate anion to form an easily ion exchangeable quaternary onium salt is not known at present, and accordingly, a complicated, costly method is disadvantageously forced to be employed for the recovery of the polyfluorocarboxylate anion by using a quaternary onium salt.
As a known method of recovering a polyfluorocarboxylic acid from an aqueous medium by using a quaternary onium salt, there can be mentioned, for example, a method of recovering a fluoroalkanoic acid from the aqueous medium used in the polymerization of a fluoroolefin by using a salt of the fluoroalkanoic acid as a surfactant as described in Japanese Patent Publication (OPI) No. 246142/1986. According to this method a quaternary ammonium salt of the fluoroalkanoic acid is firstly formed and secondly an aqueous 50% by weight sulfuric acid solution is added to the quaternary ammonium salt and thirdly the mixture solution is heated to distill the fluoroalkanoic acid. This method is complicated and moreover, requires the treatment of the quaternary onium salt in an aqueous highly concentrated sulfuric acid solution at a high temperature of at least 100.degree. C. for a long period of time and the decomposition of the part of the quaternary onium cation cannot be avoided. Thus, this method cannot be said to be economically advantageous.
On the other hand, when an effective method of treating a stable ion pair of Q.sup.+ and a polyfluorocarboxylate anion is established, a process for economically recovering a polyfluorocarboxylic acid by using a quaternary onium salt will be possible.
(2) Activation of the catalyst in a phase transfer catalytic reaction
In the phase transfer catalytic reaction using a quaternary onium salt as the catalyst, when RfCO.sub.2.sup.- is formed as the main product or as the by-product, a stable ion pair of RfCO.sub.2.sup.- Q.sup.+ is formed and the ion exchange-ability of the catalyst is decreased to suppress the activity of the phase transfer catalytic reaction. In such a case in order to economically conduct the phase transfer catalytic reaction by effectively using Q.sup.+, it is desired to develop an economical method to activate the quaternary onium salt with decreased catalytic activity to form an easily ion exchangeable and highly active quaternary onium salt.
As the method of converting a quaternary onium salt forming a stable ion pair to an easily ion exchangeable quaternary onium salt, there are known, for example, a method using dimethylsulfuric acid by Branstrom (A. Branstrom, "Preparative Ion Pair Extraction", pp 139-148, Apotekarsocieteten/Hassle, Lakemedel, Sweden, 1974) and a method using silver oxide [W. T. Ford, Journal of American Chemical Society, 95, 7381 (1973)]. However, these methods which use toxic reagents and expensive reagents and whose procedures are complicated cannot be said to be economically advantageous because of the high cost of the treatment.